Actually one can make a guide out of 3/4 ply to guide a hole saw..take the center bit out and clamp the sheet metal between the guide and another piece of ply and do that..but then if your using 20 gauge metal just scribe a circle and use a pair of aircraft snips and make your piece..a bit of filing and you are home free..Either way works fine. tho I do the snips deal most days..

Sam

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I have tried most all of it and now do what is known to work..